Regenerative mode locked laser swept source for OCT medical imaging

Abstract

An optical coherence analysis system uses a laser swept source that is constrained to operate in a mode locked condition using regenerative mode-locking. This is accomplished by synchronously changing the laser cavity's net gain and / or phase based on time varying intensity of the swept optical signal generated by the laser. This produces a stable pulsation behavior, which is associated with smooth tuning (low optical frequency reference clock jitter) and low relative intensity noise (RIN).

Description

BACKGROUND OF THE INVENTION[0001]Optical coherence analysis relies on the use of the interference phenomena between a reference wave and an experimental wave or between two parts of an experimental wave to measure distances and thicknesses, and calculate indices of refraction of a sample. Optical Coherence Tomography (OCT) is one example technology that is used to perform high-resolution cross sectional imaging. It is often applied to imaging biological tissue structures, for example, on microscopic scales in real time. Optical waves are reflected from an object or sample and a computer produces images of cross sections of the object by using information on how the waves are changed upon reflection.[0002]Fourier domain OCT (FD-OCT) currently offers the best performance for many applications. Moreover, of the Fourier domain approaches, swept-source OCT has distinct advantages over techniques such as spectrum-encoded OCT because it has the capability of balanced and polarization diver...

AI Technical Summary

Benefits of technology

[0011]The present invention concerns a swept tunable laser source. During its swept operation, it is constrained to operate in a controlled mode locked regime by using the time-varying intensity of the swept optical signal as feedback to control the mode-locked operation of laser swept source. In the illustrated embodiments, this is accomplished by actively modulating cavity gain and / or intracavity elements. This has the effect of stabilizing the emission characteristics of the laser and avoids noisy disruptions due to uncertainty or flips in the number of pulses circulating in the cavity. Instead, the mode locking system stabilizes the pulsation behavior of the laser by modulating a gain, for example, of the cavity of the laser at a harmonic of the cavity round trip frequency that is instantaneously measured by time-varying intensity of the swept optical signal as feedback to control the mode-locked operation of laser swept source.

[0012]In other embodiments described below, the stabilization is accomplished by modulating an intracavity phase modulator or a lossy element within the cavity. In still other embodiments, stabilization is facilitated with an intracavity saturable absorber, for example. As a result, in some cases operation in a stable mode locked regime can facilitate not only the tuning to lower optical frequencies, but also high speed tuning to higher optical frequencies to thereby enable stable and smooth up and down tuning.

Problems solved by technology

Certain tradeoffs in laser design, however, can be problematic for OCT systems.

Short laser cavities, however, create problems in terms of the spectral spacing of the longitudinal cavity modes of the laser.

This results in greater mode hopping noise as the laser is tuned over these discrete cavity modes.

The drawback of these devices is their complexity, however.

Moreover, the ring cavity including the long storage fiber creates its own performance problems such as dispersion and stability.

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